CKSUM(1) OpenBSD Reference Manual CKSUM(1)
NAME
cksum, sum - display file checksums and block counts
SYNOPSIS
cksum [-p | -t | -x | -c [checklist ...] | -s string | file ...]
[-a algorithm] [-o 1 | 2]
sum [file ...]
DESCRIPTION
The cksum utility writes to the standard output a single line for each
input file. The format of this line varies with the algorithm being used
as follows:
cksum The output line consists of three whitespace separated fields: a
CRC checksum, the number of octets in the input, and name of the
file or string. If no file name is specified, the standard input
is used and no file name is written.
sum The output line consists of three whitespace separated fields: a
CRC checksum, the number of kilobytes in the input, and name of
the file or string. If no file name is specified, the standard
input is used and no file name is written.
sysvsum
The output line consists of three whitespace separated fields: a
CRC checksum, the number of 512-byte blocks in the input, and
name of the file or string. If no file name is specified, the
standard input is used and no file name is written.
all others
The output line consists of four whitespace separated fields: the
name of the algorithm used, the name of the file or string in
parentheses, an equals sign, and the cryptographic hash of the
input. If no file name is specified, the standard input is used
and only the cryptographic hash is output.
The sum utility is identical to the cksum utility, except that it de-
faults to using historic algorithm 1, as described below. It is provided
for compatibility only.
The options are as follows:
-a algorithm
Use the specified algorithm instead of the default (cksum). Sup-
ported algorithms include cksum, md4, md5, rmd160, sha1, sha256,
sha384, sha512, sum, and sysvsum. Algorithms specified by the -a
option are case-insensitive.
-c [checklist ...]
Compares all checksums contained in the file checklist with newly
computed checksums for the corresponding files. Output consists
of the digest used, the file name, and an OK or FAILED for the
result of the comparison. This will validate any of the support-
ed checksums. If no file is given, stdin is used.
-o 1 | 2
Use historic algorithms instead of the (superior) default one
(see below).
-p Echoes stdin to stdout and appends the checksum to stdout.
-s string
Prints a checksum of the given string.
-t Runs a built-in time trial.
-x Runs a built-in test script.
Algorithm 1 (aka sum) is the algorithm used by historic BSD systems as
the sum algorithm and by historic AT&T System V UNIX systems as the sum
algorithm when using the -r option. This is a 16-bit checksum, with a
right rotation before each addition; overflow is discarded.
Algorithm 2 (aka sysvsum) is the algorithm used by historic AT&T System V
UNIX systems as the default sum algorithm. This is a 32-bit checksum,
and is defined as follows:
s = sum of all bytes;
r = s % 2^16 + (s % 2^32) / 2^16;
cksum = (r % 2^16) + r / 2^16;
Both algorithm 1 and 2 write to the standard output the same fields as
the default algorithm, except that the size of the file in bytes is re-
placed with the size of the file in blocks. For historic reasons, the
block size is 1024 for algorithm 1 and 512 for algorithm 2. Partial
blocks are rounded up.
The default CRC used is based on the polynomial used for CRC error check-
ing in the networking standard ISO 8802-3: 1989 . The CRC checksum en-
coding is defined by the generating polynomial:
G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
Mathematically, the CRC value corresponding to a given file is defined by
the following procedure:
The n bits to be evaluated are considered to be the coefficients of
a mod 2 polynomial M(x) of degree n-1. These n bits are the bits
from the file, with the most significant bit being the most signif-
icant bit of the first octet of the file and the last bit being the
least significant bit of the last octet, padded with zero bits (if
necessary) to achieve an integral number of octets, followed by one
or more octets representing the length of the file as a binary val-
ue, least significant octet first. The smallest number of octets
capable of representing this integer are used.
M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided
by G(x) using mod 2 division, producing a remainder R(x) of degree
<= 31.
The coefficients of R(x) are considered to be a 32-bit sequence.
The bit sequence is complemented and the result is the CRC.
The other available algorithms are described in their respective man
pages in section 3 of the manual.
DIAGNOSTICS
The cksum and sum utilities exit 0 on success or >0 if an error occurred.
SEE ALSO
md5(1), rmd160(1), sha1(1)
The default calculation is identical to that given in pseudo-code in the
following ACM article:
Dilip V. Sarwate, "Computation of Cyclic Redundancy Checks Via Table
Lookup", Communications of the ACM, August 1988.
STANDARDS
The cksum utility is compliant with the IEEE Std 1003.2-1992
(``POSIX.2'') specification.
HISTORY
A sum command appeared in Version 2 AT&T UNIX. The cksum utility ap-
peared in 4.4BSD.
WARNING
Do not use the cksum, md4, sum, or sysvsum algorithms to detect hostile
binary modifications. An attacker can trivially produce backdoored dae-
mons which have the same checksum as the standard versions. Use a cryp-
tographic-strength checksum (such as MD5) instead.
OpenBSD 3.6 April 30, 2004 3
|
